The instant N-oxyl compounds can be prepared in a number of ways. One method is described in U.S. Pat. No. 4,665,185 where a hindered amine having an N--H moiety is oxidized with a hydroperoxide in the presence of an appropriate metal carbonyl, oxide or alkoxide catalyst, such a tert-butyl hydroperoxide in the presence of molybdenum trioxide catalyst.
The oxidation of a hindered amine with hydrogen peroxide in the presence of sodium tungstate catalyst is reported by V. Kaliska et al., Chem. Pap., 42 (2), 243-8 (1988)[Chem. Abst. 109, 92734n (1988)]. The use of ultrasound facilitates the reaction when the hindered amine has a fatty acid group at the 4-position of the piperidine ring.
The preparation of the instant N-oxyl compounds has also been described by the transesterification of the lower alkyl ester of a carboxylic acid with the 1-oxyl hindered amine alcohol as taught by I. Dr agutan, Free Radical Res. Comm. 9 (3-6), 379 (1990) and by V. D. Sholle et al., Izv. Akad. Nauk. SSSR, Ser. Khim, 11, 2578 (1981)[Chem. Abst. 96, 84950c (1982)] using sodium methoxide or sodium ethylate as catalyst. It is noted that these catalysts including lithium amide cause severe decomposition and the formation of oxidation-reduction by-products in many of the reactions causing little of the desired N-oxyl ester product to form.
U.S. Pat. No. 5,218,116 teaches that hindered amines can be oxidized to their N-oxyl derivatives with hydrogen peroxide in the presence of a titanium catalyst which is a titanium silicalite or synthetic zeolite. EP 574,667 A1 teaches that divalent metal catalysts are useful for the same purpose.
Russian (SU) patent No. 1,168,556 describes the preparation of 1-oxyl-2,2,6,6-tetramethylpiperidin-yl esters of carboxylic acids by reaction of the corresponding 4-hydroxy compound with a lower alkyl ester of a mono- or di-carboxylic acid at 125.degree.-140.degree. C. in the presence of a tetraalkyl orthotitanate transesterification catalyst in xylene. The yields are reported to vary from 51 to 84%. The patent further states that, when the reaction is run at a temperature below 125.degree. C., the yields obtained are considerably lower.
In actuality, due to the reactive nature of the N-oxyl group, using the conditions described in the Russian reference, namely in xylene at 140.degree. C., the desired N-oxyl product is produced in low yield accompanied by a host of various by-products.
Although the Russian reference genetically describes a method for making such N-oxyl ester products, discrete chemically significant improvements in said process have resulted in an improved process for making the instant N-oxyl ester products in higher yields and with excellent purity at lower temperature and with more benign solvent systems.